Once in a Blue Moon

The following post was written by Professor Stephen Kane, who has been researching planets around other stars for almost 20 years and has discovered and characterized hundreds of exoplanets, including Kepler-186f - which is the smallest planet yet to have been found in the Habitable Zone of a star. After spending many years working at the NASA Exoplanet Science Institute, Kane is now a Professor of Astrophysics at San Francisco State University.

In previous blog posts I described the concept of the Habitable Zone and how it ties into the story of Civilization: Beyond Earth and its expansion, Rising Tide. The Habitable Zone is the region around a star where liquid water could be on the surface of a planet. This definition applies to planets that are in independent orbits around their star and so only accounts for the energy received via starlight. However, for large moons there are some additional sources of energy due to interaction with the planet they are orbiting. The planet can be a source of heat and reflected light from the star. Mostly though, the gravity of the planet will provide enormous amounts of energy to the moon, sometimes more than the energy that the moon receives from the star.

We can see several great examples of this in our own Solar System. The giant planet Jupiter has four large moons (known as the Galilean moons since they were first observed by Galileo Galilei) called Io, Europa, Ganymede, and Callisto. The moons range in size from Ganymede, the largest moon in the Solar System, to Europa, slightly smaller than Earth's moon.

Jupiter and its moons are five times further away from the Sun than the Earth and well outside the boundaries of the Sun's Habitable Zone. Based on that alone, we would normally assume the moons of Jupiter to be frozen solid with no chance of having the capacity to harbor life. However, Io is the most volcanically active body in the entire Solar System with volcanoes constantly erupting and covering the surface with sulfur. There is also strong evidence to show that Europa has a significant liquid water ocean under the surface ice that contains more water than all of the Earth's oceans combined. How can such small objects so far away from the Sun be geologically active?

The answer lies in the effect of Jupiter's gravity on the moons. Io and Europa are both very close to Jupiter and so they receive something called tidal energy from Jupiter. This energy is the gravity of Jupiter competing with the gravity of the outer moons resulting in a tug-of-war on Europa and, particularly, Io. The effect is similar to the heating of a tennis ball when you squeeze it in your hand. The tidal energy combines with the heat and reflected light from Jupiter as well as the sunlight received directly from the Sun to provide more than enough energy to ensure that the moons of Jupiter remain active for many years to come.

What does all of this have to do with habitability? Energy received by moons from their host planets means that there are pockets of habitability that can occur outside of the traditional Habitable Zone. Since moons are more common than planets in our own Solar System, we have good reason to think that moons in general may also be very common. Ignoring these objects may mean that we miss opportunities to search for life in abodes that are perfect for life to not only exist, but flourish in.

There can even be cases where a giant planet is in the Habitable Zone so that the moons receive sufficient energy to retain liquid water independent of the energy received from the planet. A useful thought experiment is to consider how the Galilean moons would be different if Jupiter and its entire system of moons were moved to Earth's orbit. Although we (thankfully!) do not have a giant planet where Earth's orbit is in our Solar System, we do see examples of such planets in other systems. The bright star mu Ara has four known planets, three of which are portrayed in the below figure. Planet b is a giant planet almost twice as massive as Jupiter and so probably has a large system of moons orbiting it, some of which could be even larger than Ganymede. The most interesting part though is where the planet orbits. The figure below shows that the planetary orbit lies right in the middle of the Habitable Zone, ensuring that even though the planet is likely uninhabitable, the moons of the planet may be teeming with life.

It will be some years before we know for sure if there is life on so-called exomoons, but in the meantime they have served as an inspiration for many science fiction writers. In the Star Wars franchise, the moons of the giant planets Yavin (in A New Hope) and Endor (in Return of the Jedi) serve as primary locations for much of the action. In the film Avatar, the writers created a fictional giant planet Polyphemus that is orbited by a habitable moon called Pandora (see picture below). Clearly such a habitat is seen as feasible by writers and it appears increasingly possible from recent discoveries that the writers may have been correct all along.

When humans leave the Earth in the story of Civilization: Beyond Earth, it isn't exactly specified whether the humans find their new home on an exoplanet or an exomoon. The range of possible exomoon surfaces is undoubtedly just as diverse as those for terrestrial planets, resulting in desert, icy, water, and even jungle-covered surfaces. So the next time you are building a civilization in the world of Beyond Earth, take a moment to imagine that the inhabitants of your civilization may look to the sky and see a giant planet hovering majestically overhead.